Geological sequestration of CO2 is currently being studied as a possible method for mitigating the rapid rise of greenhouse gases in the atmosphere. For example, CO2 might be sequestered in the permeable layers of formations associated with oil and gas wells. Such the permeable layers are typically located beneath an impermeable layer which form a natural barrier against upward movement of the CO2. Well boreholes provide a pathway for moving CO2 into the permeable layer. However, it is possible for leakage pathways to form through the cement annulus between the well casing and the formation. Cement, in a multitude of reaction steps, has been demonstrated to deteriorate and form CaCO3 in the presence of CO2 and water (see Ch. 7 Special Cement Systems, by E. B. Nelson et al., Cement Handbook, section on Cements for Enhanced Oil Recovery by CO2-Flooding). In order for long term CO2 storage to be practical, relatively little of the injected gas can be permitted to leak back into the atmosphere (see IPCC's special report on carbon dioxide capture and storage, pg 197, 2006). It is therefore desirable and important to know the quality of the cement in a formation selected for CO2 sequestration, both before and after injection of CO2.
Until now, formation tests have been designed to measure the permeability of a reservoir. Although quantifying skin is a common practice in well testing, and it may be appealing to regard cement as a skin, conventional skin estimation procedures work only when skin is sufficiently transmissible, i.e., the skin zone permeability is not orders of magnitude smaller than that of the formation. The reason for this is the skin zone is treated as being in pseudo-steady state, i.e., pressure drop across the skin region is directly related to flux (van Everdingen, A. F. 1953, The Skin Effect and its Influence on the Productive Capacity of a Well, Trans. AIME, 198, 171-176). Consequently, existing techniques are not entirely suited to estimating degradation of cement.